New insights on the occurrence of peperites and ... · Abstract. The PMP (Paraná Magmatic...

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New insights on the occurrence of peperites and sedimentarydeposits within the silicic volcanic sequences of the ParanáMagmatic Province, Brazil

A. C. F. Luchetti1, A. J. R. Nardy1, F. B. Machado2, J. E. O. Madeira3, and J. M. Arnosio4

1Instituto de Geociências e Ciências Exatas, Universidade Estadual Paulista, Rio Claro, SP, 13506-900, Brazil2Departamento de Ciências Exatas e da Terra, Universidade Federal de São Paulo, São Paulo, SP, 09920-540, Brazil3Departamento de Geologia da Faculdade de Ciências (GeoFCUL), Instituto Dom Luiz, Universidade de Lisboa, Lisboa,Portugal4Instituto Geonorte, Facultad de Ciencias Naturales, Universidad Nacional de Salta, Salta, Argentina

Correspondence to:A. C. F. Luchetti ([email protected])

Received: 14 November 2013 – Published in Solid Earth Discuss.: 16 December 2013Revised: 29 January 2014 – Accepted: 29 January 2014 – Published: 10 March 2014

Abstract. The PMP (Paraná Magmatic Province) is charac-terized by lava flows of the Early Cretaceous Serra Geral For-mation which covers about 75 % of the Paraná Basin (south-ern and southeastern Brazil), composed of a thick (up to1600 m) volcanic sequence formed by a succession of pet-rographically and geochemically distinct units of basic andsilicic composition. The whole package must have been em-placed during approximately 3 million years of nearly un-interrupted activity. A few aeolian sandstone layers, indicat-ing arid environmental conditions (the Botucatu Formation),are interlayered in the lower basalts. Above the basalts, thePalmas and Chapecó Members are composed of silicic vol-canic rocks (quartz latites, dacites, rhyodacites and rhyolites)and basalts. This paper presents new evidence of sedimen-tation episodes separating silicic volcanic events, expressedby the occurrence of sedimentary deposits. Interaction be-tween the volcanic bodies and the coeval unconsolidated sed-iments formed peperites. The sediments were observed be-tween basaltic lava flows and silicic rocks or interlayered inthe Palmas-type rocks, between the Chapecó-type rocks andoverlying basaltic flows, between silicic bodies of the Pal-mas and Chapecó types, and interlayered within Palmas-typeunits. The observed structures indicate that the sedimentswere still wet and unconsolidated, or weakly consolidated,at the time of volcanism, which, coupled with the sedimentfeatures, reflect environmental conditions that are differentfrom those characterizing the Botucatu arid conditions.

1 Introduction

The Early Cretaceous Serra Geral Formation is the resultof a major volcanic phase that covered about 917 000 km2,about 60 % of the surface of the Paraná Basin (Frank et al.,2009; Fig. 1). Three main petrographic types can be distin-guished from macroscopic observation of these rocks. Themost common type are basalts, presenting predominant inter-granular texture and its variations, including subophitic, in-tersertal and hialophitic. The other two types of silicic rockspresent massive and aphyric textures (Palmas type – ATP)and porphyritic textures (Chapecó type – ATC).

Macroscopic characteristics allowed easy separation in thefield of these two members of the Serra Geral Formation andtheir geological mapping (Bellieni et al., 1983; Piccirillo etal., 1988). The Palmas and Chapecó rock types occur in asso-ciation with basaltic flows that are more common near the topand bottom of these two lithostratigraphic units. Geologicalmapping also shows that the Palmas and Chapecó Memberscover 63 000 km2, in the states of Paraná, Santa Catarina andRio Grande do Sul. The volume of the two members amountsto approximately 14 500 km3, which corresponds to 2.5 % ofthe total volume of the Serra Geral Formation (Nardy et al.,2002, 2008). Geochronological dating by40Ar / 39Ar showsthat the age of volcanic rocks of the Serra Geral Forma-tion ranges from 133.6 to 131.5 Ma in its northern sector,and from 134.6 to 134.1 Ma in the southern (Renne et al.,1992, 1996a, b; Turner et al., 1994; Ernest et al., 1999, 2002;

Published by Copernicus Publications on behalf of the European Geosciences Union.

122 A. C. F. Luchetti et al.: Peperites and sedimentary deposits within the silicic volcanic sequences of the PMP

Fig. 1. Map of the Paraná Basin with the location of the acidic members of the Serra Geral Formation according to Nardy et al. (2008).Legend: 1 – pre-volcanic sedimentary rocks; 2 – basalts (the Serra Geral Formation); 3– silicic(a) Chapecó and(b) Palmas members (theSerra Geral Formation); 4 – sedimentary post-volcanic sequences (Bauru Basin); 5 – anticline structures; 6 – syncline structures; 7 – oceaniclineaments; 8 – continental lineaments.

Mincato et al., 2003; Thiede and Vasconcelos, 2010; Pintoet al., 2010). More recently, Janasi et al. (2011), using U / Pbratios from baddeleyite/zircon crystals determined by isotopedilution thermal ionization mass spectrometry from rocks ofthe Chapecó Member, obtained an age of 134.3± 0.8 Ma,compatible with previous age determinations. However, agesobtained in the basaltic flows indicate the duration of the vol-canism to be around 3 Ma, which is consistent with paleo-magnetic data presented by Ernesto and Marques (2004).

Thus far, the presence of sediments (sandstones of the Bo-tucatu Formation) intercalated in the volcanic sequence wasonly reported in the lower basaltic pile. These consist of sandbodies presenting aeolian structures such as bypass surfaces,single dunes, sand-filled cracks and multi-dune ergs (Jerramand Stollhofen, 2002; Petry et al., 2007; Waichel et al., 2008),representing a desert environment that persisted during thevoluminous initial phase of basaltic volcanism. In this work,

the occurrence of sedimentation and development of associ-ated peperites in the final stage of the PMP (Paraná MagmaticProvince) volcanic event is presented. The sediments, pre-dominantly sandy–silty, and unrelated to the Botucatu For-mation, indicate a change in the environmental conditions inthe Paraná Basin, and attest to the occurrence of significantperiods of quiescence during the final stages of the magmaticactivity.

2 Petrographic and geochemical aspects of silicicvolcanic rocks

The Palmas-type silicic volcanic rocks (ATP) are charac-terized by light-gray to brownish-red color, hypohyaline–holohyaline, aphyric to micro-porphyritic textures and astriking salt-and-pepper aspect, with some holohyalineunits (pichstones) displaying black color, glassy luster and

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A. C. F. Luchetti et al.: Peperites and sedimentary deposits within the silicic volcanic sequences of the PMP 123

prominent conchoidal fractures. However, due to its amor-phous nature, the glass is easily altered and thus in most out-crops the rock is completely weathered, presenting a brown-ish color and (often resembling sedimentary deposits) dot-ted with abundant vesicles and quartz-filled amygdales upto 10 mm in length. The mineralogy is composed of micro-phenocrysts (granularity smaller than 0.2 mm) of plagioclase(labradorite) – which comprise up to 16 % of the total volumeof the rock – 11 % of augite, 3 % of pigeonite, 5 % of mag-netite and less than 1 % of apatite. These crystals may ex-hibit rapid cooling structures (quenching), developing skele-tal, lath and hollow shapes, or swallowtail terminations. Thematrix reaches 63 % of the rock volume on average, and iscomposed of dark-brown, slightly birefringent glass, char-acterized by a granophyric texture of abundant microlites,and alkali feldspar and quartz intergrowth that surrounds thecrystal phases.

The Chapecó-type silicic volcanic rocks (ATC) are por-phyritic, with an average of 24 % of plagioclase phenocrystsup to 2 cm long, in a light-gray (when fresh) to brown (whenweathered) aphanitic matrix. The mineralogy consists of eu-hedral andesine phenocrysts in a matrix composed of 4.5 %of augite, 2.2 % of pigeonite, 3.7 % of magnetite, and 1.7 %of apatite (average composition) surrounded by quartz andalkali feldspar fabric in felsitic, locally granophyric, arrange-ment (vitrophyric texture).

The chemical composition of the silicic volcanic rocks ofthe PMP, according to Nardy et al. (2008), show two maingroups that may be observed in an R1× R2 diagram (Fig. 2;De La Roche et al., 1980). The first one, the low-Ti suite, be-longs to the tholeiitic field (tholeiitic basalts, andesi-basaltsand andesites) associated with the Palmas-type silicic vol-canic rocks, which belong to the rhyodacite and rhyolitefields. The second group, the high-Ti suite, is displaced to-wards the transitional field (transitional basalt, lati-basalt andlatites). The Chapecó-type silicic volcanic rocks belong tothis group in the rhyodacite and quartz latite fields (Fig. 2).The bulk-rock representative compositions for both Palmasand Chapecó types are listed in Table 1.

According to Bellieni et al. (1984a) the chemistry ofthe volcanic rocks and their spatial distribution allow theParaná Basin to be schematically subdivided into three mainregions: (1) southern, encompassing the tholeiitic suite inthe southern Uruguay River alignment; (2) northern, wheretholeiitic-transitional rocks occur in the northern PiquiriRiver alignment; and (3) central, located between the Piquiriand Uruguay River alignments, where both rock types arepresent (Fig. 1). The spatial distribution of the tholeiitic– ATP and transitional-tholeiitic-ATC suites indicates thatacidic volcanic rocks may have been derived from the as-sociated basalts, or ATC melts are derived from tholeiitic-transitional basalts and ATP from tholeiitic basalts as sug-gested by Bellieni et al. (1986) and Garland et al. (1995).

Fig. 2. R1× R2 diagram (De La Roche et al., 1980).(A) Tholeiiticsuite (Low-Ti basalts and ATP), and(B) tholeiitic-transitional suite(High-Ti basalts and ATC).

3 Stratigraphy

The 1600 m thick PMP volcanic sequence consists of up to32 lava flows of predominantly basic to intermediate com-positions (tholeiitic basalts, andesi-basalts and andesites), aswell as felsic volcanic rocks (dacites, rhyodacites and rhyo-lites; Bellieni et al., 1984, 1986).

The base of the stratigraphic column is composed of athick sequence of basic to intermediate flows that overlapthe aeolian sandstones of the Botucatu Formation. The sand-stones may also occur interlayered in the first hundred me-ters of the basaltic pile. The Palmas and Chapecó Membersoverlap the basalt flows. The Palmas Member is character-ized by silicic volcanic bodies (ATP type) associated with afew basaltic lava flows and crops out from the central regionof the basin southwards, where it may reach a thickness of270 m. The Chapecó Member, composed of silicic volcanicrocks (ATC type), occurs in the northern and central regionsof the Paraná Basin, the largest thickness, reaching 250 m,being present in the central region. This Member overlapsthe basalts, but in the northern portion of the basin (Parana-panema River region – SP), it is found directly on the sand-stones of the Botucatu Formation.

In the center of the basin the two silicic Members overlap,indicating that the Palmas Member is older than Chapecó,although ATP-type rocks may be found interlayered in theChapecó Member.

The last pulses of PMP volcanism emplaced basalt flowsthat cover both the Palmas- and Chapecó-type rocks and be-come thicker towards the northern area of the basin.

3.1 Peperites and sedimentary deposit

The literature (Marques and Ernesto, 2004; Thiede and Vas-concellos, 2011) indicates that the PMP magmatism occurredquickly, during a time interval that did not exceed 3 Ma,and in a rather continuous way, based on a few occurrencesof sedimentary intercalations or paleosoils within the vol-canic sequence. However, recent field work has revealed the

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Table 1. Representative analysis of Palmas and Chapecó silicic volcanics of the PMP (major elements, in oxide wt %; trace elements, inppm).

Palmas Chapecó

Sample KC KSE KSE KS KSU KC PU KNO

No. 505 406 419 319 237 482 1011 442SiO2 65.48 66.44 67.58 68.70 70.46 64.38 65.44 66.93TiO2 1.11 0.96 0.95 0.95 0.71 1.46 1.57 1.24Al2O3 12.96 12.74 12.35 12.12 12.38 12.83 13.03 13.03Fe2O3 6.78 6.15 6.15 5.69 5.22 7.01 7.57 6.54MnO 0.08 0.11 0.20 0.10 0.09 0.16 0.12 0.10MgO 1.08 1.70 1.22 1.28 0.61 1.36 1.28 0.75CaO 2.78 2.93 3.09 2.68 2.20 2.91 2.94 2.07Na2O 3.64 2.87 2.78 2.30 2.92 3.32 3.61 3.36K2O 4.01 3.89 4.17 4.69 4.74 4.45 4.33 4.61P2O5 0.33 0.27 0.26 0.26 0.20 0.48 0.46 0.33LOI 1.93 1.91 1.01 0.99 0.57 1.05 0.61 1.63SUM 100.17 99.98 99.75 99.76 100.09 99.41 100.96 100.60

Cu 128 75 78 63 18 7 9 14Ni 7 8 7 8 3 4 5 5Ba 610 706 694 588 613 1076 1003 1199Rb 169 160 165 175 206 100 101 136Sr 135 143 127 137 102 360 337 318Zr 279 258 252 266 319 633 670 592Y 63 41 57 42 55 65 66 60Nb 22 20 20 21 23 48 51 44U 3.51 4.07 4.04 4.17 3.25 1.74 1.91 2.68Th 11.53 11.30 11.64 12.09 12.25 8.76 8.62 12.69La 42.0 35.0 36.1 36.0 43.2 60.4 63.7 67.8Ce 88.0 76.0 78.0 73.0 95.5 144.6 147.0 145.0Nd 46.0 36.0 34.8 34.0 42.7 68.4 71.7 70.9Sm 9.40 7.00 7.47 7.10 8.78 15.78 15.10 14.50Eu 2.02 1.66 1.53 1.53 1.60 3.60 3.52 3.11Gd 9.80 7.30 7.28 8.10 8.78 14.04 13.10 11.90Dy 8.30 6.90 7.43 7.40 8.32 10.83 11.50 10.20Ho 1.70 1.40 1.60Er 4.70 4.00 4.54 4.60 5.25 5.97 6.21 5.42Yb 4.50 3.40 3.92 3.70 4.61 4.79 5.03 4.39Lu 0.60 0.50 0.61 0.60 0.71 0.65 0.76 0.65

presence of frequent sediment lenses and peperites in variousstratigraphic levels within the silicic volcanic sequence.

Peperite is a genetic term used for rocks formed in situ bythe interaction between hot magma (intrusive bodies, lava orpyroclastic flows) and coeval wet sediments (Fisher, 1960;Williams and McBirney, 1979; White et al., 2000; Skillinget al., 2002). Nonetheless, some authors (e.g., Jerram et al.,1999; Jerram and Stollhofen) have also described an inter-action with dry sediments. Peperite is classified into twobasic types according to the shape of its elements (Busby-Spera and White, 1987):blocky, in which the volcanic clastspresent angular shapes and show a jigsaw-fit texture reflect-ing in situ quench fragmentation in a brittle state, andfluidalin which volcanic clasts present irregular, fluid (amoeboid),globular to undefined shapes, reflecting a ductile state dur-

ing fragmentation, with the sediment often filling vesiclesand being injected into cracks in the volcanic clasts/rocks,although more complex shapes may also be found (McPhieet al., 1993; Skilling et al., 2002).

Sedimentary deposits and peperites found in the Chapecóand Palmas members present a wide distribution in theParaná Basin, as shown in Fig. 3, and are described below.

In the São Jerônimo da Serra (Paraná State) region, in thenorthern sector of the Paraná Basin, ATC-type rocks over-lap a sandstone forming blocky peperite (Fig. 4). The blockypeperite is comprised of clastic dikes, which can be from afew centimeters to just over 1 m thick (Fig. 4a), and breccias,composed of matrix-supported angular to rounded volcanicblocks of variable size (Fig. 4b). The sandstone is poorlysorted with angular to rounded quartz grains (Fig. 4c and d),



Fig. 3. Map with the location of sediment and peperite outcrops(lozenges) within the Palmas and Chapecó Members: yellow – ATPunits; red – ATC units.

and it was silicified by thermal metamorphism by the overly-ing volcanic material.

In the region of Mangueirinha and Palmas (Paraná State),in the center of the basin, both ATC- and ATP-type rocks cropout as either overlying basalts or overlapping each other. Asandstone layer was observed intercalated between ATC-typesilicic body and an overlying basaltic lava flow. The base ofthe sediment is a breccia formed by vesicular ATC clasts setin a sandy matrix, implying some erosion degrees of the topof the volcanic body during sedimentation. In another loca-tion, an ATC-type silicic unit overlies a vesicular ATP-typesilicic body, with a red clayey–silty sediment intercalated be-tween the two units. The sediment was injected upwards intofractures in the overlying ATC unit forming peperite withboth fluidal and blocky features (Fig. 5a and b), while in theunderlying ATP unit, the sediment filled cooling cracks with-out any peperitic interaction (figure 5c and d). In a third ex-posure, a reddish-brown silty sediment, intercalated betweentwo ATP volcanic units, formed peperite with blocky joint-ing morphology (Skilling et al., 2002), characterized by theinjection of sediment into centimeter- to millimeter-spacedjoints in the base of the overlying volcanic unit (Fig. 6).

In the southern region of the Paraná Basin (Rio Grande doSul State), sediments and peperites were observed between

Fig. 4. Peperite from São Jerônimo da Serra:(a, b) photomicro-graphs of the immature and poorly sorted sediment (c= // polariz-ers;d =X polarizers),(c) clastic dike more than 1 m thick, anddangular to sub-angular shaped clasts set in a sandstone matrix. S –sandstone; V – volcanic rock.

Fig. 5. Silicic ATC- and ATP-type rocks (V) in contact with redclayey siltstone (S):(a, b) peperites in the base of an ATC-typerock; (c andd) fractures in the top of an ATP-type rock filled withsediment, in the central region of the Paraná Basin.

some basaltic lower units and the overlying Palmas silicicsequence. In the Santa Maria region, blocky peperite wasobserved in the base of the lowermost silicic volcanic unit.The peperite displays a well-developed jigsaw-fit texture, andclosely packed, blocky to cuneiform juvenile clast shapes ofseveral sizes, separated by an orange-colored sedimentarymaterial (Fig. 7). The sediment exhibits a high degree of bak-ing (thermal metamorphism), and the juvenile clasts show in-tense devitrification, indicating very high temperature of thevolcanic material interacting with the sediment.



Fig. 6. Blocky jointed peperite in the base of an ATP-type unit(Domingos Soares city – Paraná State, central region of the ParanáBasin): V – ATP rock; S – sediment.

Near the city of Venâncio Aires, a 10 m thick sedimentarylayer underlies an ATP silicic volcanic unit. The sedimentis a reddish-colored sandstone composed of sub-angular torounded quartz grains (Fig. 8). The volcanic unit presents awell-developed horizontal jointed base and the contact withthe sandstone is sharp, apparently lacking peperitic interac-tion (Fig. 8).

A road cut on the Soledade to Lajeado highway exposes apeperite in the base of a 25 m thick basaltic lava flow inter-layered within the ATP silicic volcanic sequence. Volcanicclasts in the peperite are vesicular and display a variety ofmorphologies, from blocky to fluidal. The sediment is a red,poorly sorted sandstone, which partially fills vesicles in thevolcanic clasts.

Eastward, in the area between Bento Gonçalves and Cam-bará do Sul (Rio Grande do Sul), all the observed peperitesare interlayered within the silicic volcanic sequence and thesedimentary material becomes finer and predominantly silty.A peperite was observed in a quarry floor in Nova Petrópo-lis, near the city of Gramado, resulting from the interactionof sediment with a thin (∼ 1 m thick) amygdaloidal basalt in-terlayered in the silicic volcanic sequence. It displays blockyjuvenile clasts with irregular shapes separated by orange toreddish, poorly sorted fine-grained sandstone. The sedimentpresents vesicles that probably resulted from volatilization ofsediment water by heating (Fig. 9).

On the Rota do Sol highway, connecting Caxias do Sulto the coast, three peperite levels were observed interlay-ered in a sequence of black and glassy volcanic units of thePalmas Member. The stratigraphically lower two are sim-

Fig. 7. Blocky peperite formed by interaction of an ATP-type vol-canic body with sediment in the Santa Maria region (Rio Grande doSul State): well-developed jigsaw-fit texture, closely packed, grayblocky to cuneiform clasts of variable sizes, separated by orangesedimentary material, displaying intense baking.

ilar and characterized by vesicular, pale to greenish andglassy volcanic clasts, presenting angular to rounded irreg-ular shapes (Fig. 10). The sedimentary material is a brown,moderate to poorly sorted siltstone that also fills vesiclesin volcanic clasts. The uppermost peperite presents juvenilevolcanic clasts with a wide variety of morphologies, the flu-idal shapes being more frequent (Fig. 11). The green andvesicular volcanic clasts are surrounded by a reddish-brownsiltstone, which also fills vesicles and fractures in volcanicclasts (Fig. 11d). It is slightly coarser and more poorly sortedthan the previous two sediments (Fig. 12), and many clasts ofthe stratigraphically lower peperite display perlitic fractures(Fig. 12c).

4 Discussion and concluding remarks

The PEMP (Paraná–Etendeka Magmatic Province) is con-sidered one of the largest LIPs (large igneous provinces)of continental crust in the world, encompassing an area ofnearly 1 million cubic kilometers (Bryan et al, 2010). Of to-tal volume of the volcanic products, 95 % are preserved in theSouth American continent, in the PMP. All volcanic material



Fig. 8. Outcrop in the Venâncio Aires area (Rio Grande do SulState), where a silicic volcanic unit of the ATP type (V) rests on areddish sedimentary layer (S) lacking peperitic interaction. On theright lower corner a photomicrograph of the sandstone (X polariz-ers).

Fig. 9.Aspects of a peperite produced by interaction of a thin basaltflow (within the silicic Palmas Member) and fine-grained sedimentnear Nova Petrópolis (Rio Grande do Sul State), southern part ofthe basin:(a–c) irregular blocky volcanic clasts (V) in a reddishsandstone matrix (S), and(d) photomicrograph showing the amyg-daloidal basalt (V) in the upper part and a poorly sorted fine-grainedsandstone in the bottom (S).

was erupted in a short period of time (∼ 3 million years)without significant interruption, as deduced from the scarcityof sediments interlayered within the volcanic sequence. Infact, and up to now, the only references to the presence ofsediments interbedded within the volcanics corresponded tolayers or lenses of sandstones (intertraps), a few centime-ters to several meters thick, from the Botucatu Formation.These occur only in the base of the lava flow pile in both theAfrican (Jerram et al., 1999, 2000; Jerram and Stollhofen,

Fig. 10.Aspects of the two lowermost peperites in the Palmas vol-canic sequence, along the Rota do Sol highway (Rio Grande do SulState): images of the lowermost peperite (a, b, andc) and of themiddle peperite(d). The volcanic clasts are pale and light or darkgreen, while the siltstone is dark brown.

Fig. 11.Peperite from the stratigraphically higher level on the sec-tion exposed in the Rota do Sol highway: green to pale volcanicclasts, displaying sub-angular to irregular morphologies, set in areddish-brown siltstone matrix, which also fills vesicles in volcanicclasts (yellow arrows ind).

2002) and South American continents (Petry et al., 2007;Waichel et al., 2008). Sandstone intertraps in rhyodacites inthe Piraju–Ourinhos region (São Paulo State; Janasi et al.,2007; Luchetti, 2010), where the thick basaltic sequence ismissing, were also known. However, until now, there wereno reports of features indicating significant time breaks inthe upper part of the PMP stratigraphic sequence.

New observations of the occurrence of sedimentary lensesand peperites, resulting from volcano-sedimentary interac-tion at the base and within the upper silicic sequences ofthe PMP are presented in this paper. Throughout the Paraná



Fig. 12. Photomicrographs of peperites from the Palmas volcanicsequence, along Rota do Sol highway: (a, b andd) volcanic glassyclasts (V) set in a brown siltstone matrix (S);(c) volcanic clast dis-playing concentric fractures – perlitic texture - common in the twolowermost peperites; (e and f) detail of the poorly sorted, slightlycoarser siltstone from(d) (a, b, c, d and e – // polarizers;f – X

polarizers; P = plagioclase).

Basin these were observed associated with silicic and basalticunits of the Palmas and Chapecó Members.

Some works on the PMP report peperites derived from thebasalt-flow–sediment interaction (Petry et al., 2007; Waichelet al., 2007, 2008), the most common types found in world-wide volcanic sequences. Nevertheless, structures observedin this study reveal that this interaction also occurred be-tween silicic extrusive bodies and wet sediment. These “sili-cic peperites” are mostly blocky, highlighting a more viscousmagma (Dadd and Wagoner, 2002). However, they may alsodisplay fluidal features, which indicate less viscous melts,likely due to higher temperatures (> 1000◦C; Piccirillo andMelfi, 1988).

The sediments display immature features, suggesting lim-ited transport and, depending on the location, range frommoderately to poorly sorted sandstones to siltstones. Therewas often lava–sediment interaction producing peperites orsediment deformation by the weight of overlying volcanicunits. Evidence for the presence of wet and unconsolidatedor poorly consolidated sediments is the variety of morpholo-gies found, injections of sediment into fractures, vesicles inthe juvenile clast filled by sediment and vesiculated sediment(Skilling et al., 2002).

Paleoenvironment climatic conditions at the time of thePMP volcanism were quite dry, since the Serra Geral For-mation overlies the Botucatu Formation, described as a dryaeolian system (Sherer, 2000). Sandstone lenses up to 20 mthick interlayered in the first basalt lava flows (Jerram et al.,1999, 2000; Jerram and Stollhofen, 2002; Petry et al., 2007;Waichel et al., 2008) reveal that this system remained activeduring the magmatic event. However, new evidence of wetand locally silty sediment between basalt and silicic unitsand interlayered in the silicic units (end of the volcanic se-quence) presented here reflect a paleoenvironmental change,from a dry climate in the beginning of the PMP volcanic ac-tivity to a more humid environment (fluvial–lacustrine) dur-ing the latest phases of the magmatism, as also evidenced byWaichel et al. (2007) in peperites found in some basalt unitsof the PMP in contact with lacustrine sediments. In the SãoJerônimo da Serra region, the northern area of the basin, thechange must have started before the volcanism. Furthermore,this seems to be common in this type of setting in which hu-mid and dry aeolian systems may alternate over time due toclimate changes (Assine et al., 2004).

The deposition of sediments must have taken place in de-pressed portions of the paleorelief (small valleys or depres-sions of the original volcanic morphology), and suggests adecrease in eruptive frequency towards the end of the vol-canic activity, allowing time for the deposition of sedimen-tary material between individual volcanic events.

In conclusion, the PMP volcanism was not completelycontinuous, but presented significant pauses, mainly in theinitial and terminal phases. On the other hand, the occur-rence of sediments separating the top of the lower basalticsequence and the beginning of the silicic extrusions may rep-resent a pause in the volcanic activity that coincides with thecompositional change in the magmatism of the Paraná Mag-matic Province.

Acknowledgements.This study was supported by São PauloResearch Foundation (FAPESP) and National Council of Techno-logical and Scientific Development (CNPq). We thank FranciscoNegri for help in fieldwork in the central basin and Dr. Scarlato,Piergiorgio, Dr. Giacomoni, Pier Paolo and an anonymous refereefor valuable suggestions and review.

Edited by: P. Scarlato

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